We will examine the control mechanisms which operate upon electron transport and phosphorylation. We will use biological material in which rate of electron transport is driven by photoreactions and can be varied in short-time experiments by control of intensity and wavelength of light. And we purposely select the blue-green algae as material in which adenosine triphosphate (ATP) requirement of cell synthesis is necessarily high, in which almost all production of ATP is coupled to light-driven reactions, and in which metabolic machinery is subject to manipulation via previous history and preparation. Metabolic material will be used in the form of whole cells, intact but permeable cells (permeaplasts) and subcellular membrane preparations. Electron transport rate will be metered by light intensity and perturbed by wavelengths of light selected to overdrive either the oxidizing or the reducing photochemical driving reaction. Perturbations in electron transport will be measured in terms of absorption changes attributable to components of the electron transport chain and in terms of resulting effects upon reaction centers of the driving photochemical reactions. Attendant rates of phosphorylation will be estimated from changes in cellular ATP level. Hopefully ATP changes will be measured with the time resolution apparently needed to obtain expected rates of phosphorylation. We will seek the control mechanism which gives rise to two distinguishable steady states when one or the other of the two photoreactions is overdriven.